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| Format: | Preprint |
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2025
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| Online Access: | https://arxiv.org/abs/2507.03157 |
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| _version_ | 1866918229439414272 |
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| author | Narita, Yuma Yin, Wen |
| author_facet | Narita, Yuma Yin, Wen |
| contents | The axion-like particle (ALP), a pseudo Nambu-Goldstone boson that couples to two photons, has been studied extensively in recent years as a dark matter candidate. For initial field configurations in a minimal ALP model explaining the observed dark matter abundance, we need the potential height to exceed the ALP energy density at redshift $z\approx 5.5\times 10^{6}$ leading to: $$
f_ϕ\gtrsim4\times10^{13}\,{GeV}\,\biggl(\frac{10^{-18}\,eV}{m_ϕ}\biggr), $$ where $m_ϕ$ and $f_ϕ$ denote the ALP mass and decay constant, respectively. This bound is known for the ALP dark matter dominated by the homogeneous zero-momentum mode, under the requirement that coherent oscillations begin early enough to satisfy the late-forming dark matter constraint. One loop hole to evade this limit may be to introduce a large amount of the non-relativistic modes of the ALP with non-vanishing momenta. Here we show that the same limit remains valid even if nonzero-momentum modes dominate. Interestingly, when $nonrelativistic$ gradient modes prevail, the ALP behaves $relativistic$ radiation rather than matter, if it violates the limit. Moreover, if the typical momentum is sufficiently small, Baumkuchen-like domain walls form, which play an important role in understanding the transition. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_03157 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Relativistic Axion with Nonrelativistic Momenta: A Robust Bound on Minimal ALP Dark Matter Narita, Yuma Yin, Wen High Energy Physics - Phenomenology The axion-like particle (ALP), a pseudo Nambu-Goldstone boson that couples to two photons, has been studied extensively in recent years as a dark matter candidate. For initial field configurations in a minimal ALP model explaining the observed dark matter abundance, we need the potential height to exceed the ALP energy density at redshift $z\approx 5.5\times 10^{6}$ leading to: $$ f_ϕ\gtrsim4\times10^{13}\,{GeV}\,\biggl(\frac{10^{-18}\,eV}{m_ϕ}\biggr), $$ where $m_ϕ$ and $f_ϕ$ denote the ALP mass and decay constant, respectively. This bound is known for the ALP dark matter dominated by the homogeneous zero-momentum mode, under the requirement that coherent oscillations begin early enough to satisfy the late-forming dark matter constraint. One loop hole to evade this limit may be to introduce a large amount of the non-relativistic modes of the ALP with non-vanishing momenta. Here we show that the same limit remains valid even if nonzero-momentum modes dominate. Interestingly, when $nonrelativistic$ gradient modes prevail, the ALP behaves $relativistic$ radiation rather than matter, if it violates the limit. Moreover, if the typical momentum is sufficiently small, Baumkuchen-like domain walls form, which play an important role in understanding the transition. |
| title | Relativistic Axion with Nonrelativistic Momenta: A Robust Bound on Minimal ALP Dark Matter |
| topic | High Energy Physics - Phenomenology |
| url | https://arxiv.org/abs/2507.03157 |